Add support for custom storage layout

[cgewecke]

Only need to update the parser to support this but OZ uses the at keyword as a function name in certain contracts and the parser doesn't tolerate this.

(See Zeppelin CI in #905)

Have raised issue at solidity-parser/parser#123

---

Workaround

Use dependency resolution features in package.json to coerce @solidity-parser/parser to 0.20.0

npm

"overrides": {
  "@solidity-parser/parser": "^0.20.0"
}

yarn

"resolutions": {
  "@solidity-parser/parser": "^0.20.0"
}

[cgewecke]

Fixed in #905